Look closely at the graphics in the backgrounds. It seels to me the order should have been:
2-1 ---> 2-3 ---> 2-2
The beginning of 2-3 has the trees, grass, and geology of 2-1.
2-3 is an underground river/stream.
2-2 is a swamp area, and swamps are normally fed by a water source.
2-2 has the stage's boss in a room that looks like it could be the mouth of a cave.
If 2-3 were feeding directly into 4-1, then why isn't flooded or partially flooded?
Having a waterfall (4-2) in proximity to 4-1 would explain the dripping water.
And all of this is really just rationalization, but it makes sense to me.